This invention relates generally to methods and apparatus for lining or coating the interior of hollow objects, and more particularly to the lining of cast iron pipe and the like with concrete.
It is common to apply concrete or similar corrosion-resistant linings to the interior surfaces of metal pipe to prevent corrosion and rusting and the undesirable contamination of water carried by the pipe. The most practical way to apply such linings is to use a centrifugal process in which lining material is introduced into the interior of a length of pipe, and the pipe is rotated about its longitudinal axis. The rotation causes the lining material to be spread over the interior surfaces and to be compacted to produce a relatively smooth coating on the interior surfaces.
Considerable difficulty is encountered, however, in providing satisfactory concrete linings in pipe, particularly in long sections, e.g., twenty feet, of large diameter, e.g., forty inches, pipe. This is due, in part, to the inability to rotate the pipe at a sufficiently high enough speed to produce good compaction of the concrete so that shrinkage is minimized and so that voids or other defects do not result. As the concrete cures, shrinkage may also cause the lining to separate partially from the interior surfaces and permit voids or stress concentrations to develop in the lining, rendering it easily broken. Typically, concrete is introduced into the pipe by a slinger while the pipe is stationary. This necessitates using a concrete mix which is rather thick and not very flowable, i.e, somewhat dry, so that the concrete will stick to the pipe wall. The pipe is then rotated for a short period of time at a speed high enough to smooth out the concrete but low enough to avoid removing excessive water from the concrete. If too much water is removed, the concrete will not cure properly and the resulting lining will be powdery.
Conventional centrifugal lining apparatus supports the pipe section on spaced pairs of rollers which engage the peripheral surface of the pipe and which are driven to impart rotation to the pipe. It is practically impossible, however, to produce pipe which is perfectly round and balanced. Any out-of-roundness will cause the center of mass of the pipe to deviate from the access of rotation, and as the pipe is rotated, forces are produced which tend to lift the pipe from the rollers. To maintain the pipe in contact with the rollers, it is necessary to exert a downward force on the top of the pipe, as by using holddown rollers. Even with holddown rollers, as the pipe speed increases, lateral vibration and motion of the pipe due to out-of-roundness may become quite large. If the vibration becomes excessive, it may wreck the apparatus, and, in any event, a point is quickly reached where the force necessary to hold the pipe on the rollers exceeds the rim strength of the pipe. In addition, the lateral vibrations and bouncing to which the pipe is subjected interferes with the ability of the concrete mixture to spread uniformly and smoothly over the interior surface of the pipe and is detrimental to the resulting lining. As a result, the maximum speed at which the pipe may be rotated is substantially less than that desired to produce good compaction of the concrete.
It is desirable to provide pipe lining apparatus and methods which avoid these and other disadvantages of known methods and apparatus, and it is to this end that the present invention is directed.